AbstractThe complexation properties towards Bi3+of benzoazacrown ligands with five (H3BA3A) and six (H4BATA) heteroatoms and corresponding number of acetic arms have been investigated. Complexation constants were determined via potentiometric titration for BA3ABi complex (logβ=26.2) and refined for BATABi complex (logβ=31.7). According to NMR, X‐ray and EXAFS results, chelation of cation by H3BA3A appears to be outside the macrocyclic cavity both in the solid state and in the aqueous solution. This out‐cage location despite the high stability constant causes serum andin vivoinstability of complex of smaller azacrown H3BA3A in contrast to H4BATA. Whereas the larger macrocycle and four pendant arms of H4BATA provide long‐range coordination effectively shielding the cation thereby ensuring high inertnessin vitro, low accumulation of the radionuclide in the organs, and fast clearance.